An induction log device utilizes one or more coils to form fields which extend from a sonde which encloses log equipment into formation adjacent to a well borehole. As a practical matter the number of coils included in the log device has increased well above a single transmitter coil, typically guard or steering coils above and below the transmitting coil(s). Furthermore all these coils are wound on a common mandrel which comprises a part of the shell or housing of the sonde. Numerous turns are involved in each coil. The coils extend along the length of the mandrel while the ends of the coil(s) utilize small drilled holes in the supportive mandrel. The coil wires extend to the interior for connection with suitable power supplies to provide appropriate operating currents for the coils. As the depths of investigation have become greater and the number of coils has increased, the receiver signals which are required for operation of these coils have become quite small. Voltage levels measure typically in the tens of nanovolts. In view of the number of coils located on the mandrel and the fact that these coils all have a common coaxial mounting on the mandrel, there are substantial magnetic fields formed by the transmitter coils, which are concentrated in the relatively small diameter of the sonde. Indeed, the supportive mandrel is only two or three inches in diameter in most embodiments.
It is necessary to position a number of lengthwise or axial conductors along the sonde to provide current flow to and from the respective coils making up the coil array. In light of this, the current flows are very substantial in the transmitter coils and create interactive magnetic fields where the mutual inductance linking the several coils and the coil leads becomes very substantial.